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Post subject: Silverface Twin Reverb (1975)... Amp Problem....
Posted: Mon Dec 14, 2009 6:49 pm
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Hey guys,

I'm new new new to this forum and I have a problem...


I have a 1975 Twin Reverb (my pride'n'joy with big fat JBL speakers) that I brought to the shop to get rebiased and put new tubes in (4 JJ6L6 output tubes and four Tung-Sol 12AX7s preamp ones).


Anyways, I went to the store and try to run the amp, everything's fine until about 15 minutes after playing there's a sudden drop in Bass from the Reverb-Channel...

Does anyone of you had anything like that before? Could it have been caused from a faulty Output Tube?

Anyways, after it happened I went round the back of the amp to take a look at the Output Tubes, turns out one of the four tubes doesn't glow as much as the others... I tried switching it to another socket, same thing... :?:

The problem is, that I've send one matched quartet back already for one of the tubes was broken and I can't imagine them sending out another batch of faulty tubes..

please help, thanks

btw. I bought the tubes from the TubeAmpDoktor

thank you in advance :)


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Posted: Tue Dec 15, 2009 2:19 am
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Write these guys: http://www.tubeampdoctor.com/request_for_service.php?message_to=service

They'll advise and/or replace your tubes.

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Posted: Tue Dec 15, 2009 11:30 am
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sorry, not a clue. when you do find the problem, please let us know. good luck, awesome amp by the way. dutch

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Posted: Tue Dec 15, 2009 8:51 pm
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Hi John, that's a drag about the new quad. I'm sure TAD will replace them. I'm not familiar with which brand TAD is selling, but I have found that a well tested quad of Winged C tubes work very well in Twin Reverbs. How does the normal channel sound ? Maybe this is not a power tube issue at all. That glow is only the heater filaments. How do they look the instant you switch from standby to the play mode? Whatever the flash appearance is(orange-bluish), they should all be the same. Art

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Posted: Wed Dec 16, 2009 5:21 am
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aclempoppi wrote:
Hi John, that's a drag about the new quad. I'm sure TAD will replace them. I'm not familiar with which brand TAD is selling, but I have found that a well tested quad of Winged C tubes work very well in Twin Reverbs. How does the normal channel sound ? Maybe this is not a power tube issue at all. That glow is only the heater filaments. How do they look the instant you switch from standby to the play mode? Whatever the flash appearance is(orange-bluish), they should all be the same. Art


Hi, thanks for your reply,

The normal channel sounds beefy'n'juicy and great while the reverb channel sounds thin and trebly all the way even with Middle and Bass set to max...

I'm glad that the glow of the tubes has nothing to do with it, the moment I switch the amp on the tubes orangy and then constant orange glow, they are all the same..

JM


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Posted: Wed Dec 16, 2009 10:56 am
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JM, it seems all is well with the power section, if the normal channel is working as usual. I would suspect a faulty pre amp tube. You could swap v1 in place of v2. The v1 socket can be left empty for the test. You may want to use a sharpie to make a reference mark on the tube and socket base to aid in lining up the tube pins. V1 is furthest to the right as you're looking at the back of the amp. Art

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Posted: Wed Dec 16, 2009 11:59 am
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Okay guys,

I just got off the phone with the technician, and there's two problems with the amp

1. I put in a matched quartet of JJ 6L6 output tubes, problem is, there's only a bias balance on the amp which means that by turning the thingy (as you sense, I have no knowledge of this stuff at all) two tubes are running hotter, while the other two are running much colder.
What the technician offered was to modify this (since it's a dated way of biasing and in modern ages that's not the way to go anymore) in to a - what was it called again?! - Bias potentiometer/ Bias Adjustment Pot with which I can just shove a quartet in and bias it normally without any tubes running hotter than the others... I think I'm gonna go with that option because I have the quartet already and (most importantly) it makes biasing much easier and quicker.. Right now, the tubes are running too cold the tech said and measured...

2. As you all know, my amp's Reverberated channel has lost it's ability to pump the bass... The tech said that it's either a preamp tube that's blown to smithereens OR that my amp needs a recap and that the capacitors have oxidated and need recapment (or whatever one calls it)...

He'll take a good look at it on Saturday and suggest a price for it which is real nice, at last someone who doesn't want to screw you over like sooooo many other so-called techinicans...

I'll keep you posted on how much it costed and what effectively was wrong with the amp!


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